Homebrew has always used the version of Ruby which comes with OS X,¹ a design decision I decided to keep with Tigerbrew. Tiger comes with Ruby 1.8.2, built on Christmas Day, 2004, and with a version of Ruby that old I went in steeling myself for the inevitable ton of compatibility issues.

On the whole I was pleasantly surprised. Most of what Homebrew uses is provided in exactly the same form, and while there are differences that range from puzzling² to major³, pretty much everything Just Works.

Except, at first, for Pathname. Ruby’s Pathname class, which is an object-oriented wrapper around the File and Dir classes, is at the heart of Homebrew’s file management. The first time I tried to install something with the newborn Tigerbrew, I was quickly treated to a strange exception with an equally mysterious backtrace: Errno::ENOTDIR: Not a directory.

Curious, I dug in. I soon discovered that the bug occurred while Homebrew was unlinking an existing version of a package before beginning to install an upgrade. (For those not in the know, Homebrew installs software into isolated versioned prefixes. The active version of a given package is symlinked into the standard /usr/local locations.) Most of the files were linked and unlinked just fine, but a few files caused the method Pathname#unlink to throw an exception every time. Eventually I noticed a pattern — every symlink that Pathname choked on represented a directory. Once I noticed that, it clicked.

For those who don’t know, symlinks are actually treated on the filesystem level as special files containing their target as text. For most operations, symlinks transparently act as their targets. However, applications which hit the filesystem directly will see them as files — even when they point to directories. Since Pathname handles files and directories differently, handing its instance methods off to File or Dir as appropriate, the bug happened something like this:

• The #unlink method is called on a Pathname object representing a symlink to a directory.
• Pathname examines the object to see if it represents a file or directory, in order to determine whether to call File.unlink or Dir.unlink.
• In doing so, Pathname follows the symlink to its target and examines the properties of the target.
• Seeing that the target is a directory, Pathname calls Dir.unlink on the original symlink.
• Dir.unlink raises Errno::ENOTDIR because, of course, the symlink isn’t a directory.

The overridden version of the method can be found here. The rest of Tigerbrew’s current backports are in Tigerbrew’s file extend/tiger.rb, for the curious.